Donnchad was deceitfully killed by his rival among the Clann Cholmáin, Flann Sinna in 877 who then became king.
Flann Sinna (died 916), also called Flann mac Maíl Sechnaill, High King of Ireland
Cormac mac Cuilennáin of the Eóganachta of Cashel, King of Munster, with his "evil genius" and later successor Flaithbertach mac Inmainén by his side, raided Connaught and Leinster in retaliation and, according to some annals, defeated Flann.
He was the son of Niall Glúndub and Gormlaith, thus his father and both of his grandfathers—Niall's father Áed Finnliath and Gormlaith's father Flann Sinna—had been High King of Ireland.